1. Field of the Invention
The invention relates generally to marine seismic surveying. In particular aspects, the invention relates to systems and methods for conducting a reflection seismic survey.
2. Description of the Related Art
Seismic exploration is used to survey subterranean geological formations to determine the location of hydrocarbon formations within the earth. Reflection seismology is used to estimate the properties of the subsurface from reflected seismic waves. In reflection seismology, generated acoustic waves (i.e., shots) are propagated down through subterranean strata and reflect from acoustic impedance differences at the interfaces between various subterranean strata. Because many commercially viable hydrocarbon formations are located beneath bodies of water, marine seismic surveys have been developed. Marine seismic survey systems have been described in, for example, U.S. Pat. No. 6,026,059 issued to Starr.
The presence of background noise and multiple seismic signals tends to cover up the desired signals (traces) which reveal actual subsurface geological structure. As a result, it has become conventional to enhance the desired traces by collecting multiple signals having the same common mid-point (CMP). “Fold” quantifies the number of seismic traces that are recorded at a given CMP. Higher fold generally improves data quality as the traces are summed together such that the primary signal is enhanced by in-phase addition while ambient noise and interference are reduced. In 3-dimensional surveys, data is gathered by taking all seismic traces from an area around each CMP and assigning these traces to a “bin,” which is a discrete rectangular area of the surface area being surveyed. A 3-dimensional “image” of the subterranean structure can then be modeled from the bin data.
In a typical ocean bottom seismic survey system, a source vessel tows a source array through a body of water. The source array contains a number of seismic sources, such as air guns, which can create a seismic signal as known in the art. The source array produces seismic signals (shots) that are propagated down through the water and into the strata beneath the sea floor. As the seismic signals encounter the various subterranean strata, they are reflected back and are detected by one or more seismic receiver devices which record the signals and permit them to be analyzed. In an ocean bottom 3D seismic survey, it is typical to have a plurality of seismic recorders incorporated into an ocean bottom cable that is disposed in a linear fashion along the sea floor.